Standard Input and Output

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Introduction

Displaying output is a common process, and there are a variety of functions that do exactly that.

So far, I've used the printf function in every example as a way of displaying program output.

Similarly, getting input from the user (or from external files) is another common process.

To use the input and output functions, you'll need to include the Standard Input and Output header file (stdio.h) in your program, which explains why I've written #include <stdio.h> in previous examples.

The Standard Input File Stream, stdin

To input a value, the easiest way is to use a library function called scanf. There are other functions like getc and getchar (both used for inputting characters), but I find that scanf is by far the easiest to use. gets is another one that is covered in the future. scanf is actually a simplified version of a function called fscanf which requires you to specify an INPUT FILE STREAM.

An input file stream is basically a series of bytes that is transferred from the input source to the program (recall from the data types section that each character is one byte large, so a stream can be thought of as a series of characters). In C, stdin is the standard input file stream and is usually associated with the keyboard.

The Standard Output File Stream, stdout

putc, putchar, puts and printf are all used to output values. The latter is the simplest to use.

Some of these require an OUTPUT FILE STREAM which is a series of bytes that is transferred from the output source to the program.

stdout is the standard output file stream and is usually associated with the monitor.

stderr is also an output file stream but makes sure that the output, usually an error message, is seen.

getc and getchar

These functions allow you to input a character and assign it to a variable.

getc requires you to specify an input file stream like stdin, or as we'll see later, a file pointer (you don't need to know what a pointer is just yet!).

getchar is EXACTLY the same as getc with the input file stream set to stdin.

Both functions return integer values, which you could assign to variables for later use.

putc and putchar

To output a character, you can use one of these two functions.

putc requires you to specify two things - the variable whose character value should be printed out, and the file output stream.

putchar is exactly the same as putc, but with the output file stream set to stdout. You only need to specify the character value to print out.

The following example demonstrates the previous four functions...

#include <stdio.h>

int main() {
  char a,b;
  printf("Enter a 2 letter word ");
  printf("and press return: ");
  a = getc(stdin);
  b = getchar();
  printf("The first letter was: ");
  putc(a, stdout);
  printf("\nFollowed by: ");
  putchar(b);
  putchar(10);
  printf("Goodbye!\n");
}

Suppose I entered YO and pressed return. The output generated would be:

Enter a 2 letter word and press return: YO

            The first letter was: Y 

            Followed by: O 

            Goodbye!

After declaring two character variables, I used two printf statements to display the first line of text (printf is discussed in the next section!), simply because I was running out of space!

AS SOON AS the user enters a character, it gets assigned to the variable a because the getc function was INVOKED - a term meaning the calling of a function. Noticed how the standard input stream, stdin had to be PASSED into getc. Values passed into a function are called ARGUMENTS - a function sometimes needs extra information for it to work.

As soon as the user enters a second character, it gets assigned to b - notice how getchar requires no arguments.

The program continues when you press return - any additional characters after the second aren't stored.

The putc function takes a char or int variable as its first argument, followed by the standard output stream, stdout and prints out the specified character.

putchar only requires one argument, and also prints out the specified character.

As a reminder, \n is the NEWLINE character and is the equivalent to a carriage return. Is has an ASCII code of 10, so putchar(10) inserts a newline before printing "Goodbye!".

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